The kinetics of thermal decomposition of ammonium yttrium fluoride intermediates, (NH(4))(3)Y(2)F(9) and NH(4)Y(2)F(7) were studied by a non-isothermal thermogravimetric (TG) technique at different heating rates. Kinetic parameters were determined from the TG curves by using two model-free methods, i.e. Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). The activation energy (E) for the decomposition of (NH(4))(3)Y(2)F(9) was dependent on the fraction converted, alpha; its value was 152 kJ/mol (for alpha = 0.1-0.3) and 232 kJ/mol (for alpha = 0.4-0.8) from both models; and the pre-exponential factor (ln(A)) was 43 (for alpha = 0.1-0.3) and 59 (for alpha = 0.4-0.8). For the decomposition of NH(4)Y(2)F(7), the activation energy (E) was independent of the fraction converted, alpha, and its value was 123 kJ/mol; the pre-exponential factor (ln(A)) was 31. The decomposition of (NH(4))(3)Y(2)F(9) proceeds by one-dimensional diffusion (for alpha = 0.1-0.3), followed by two-dimensional diffusion (for alpha = 0.4-0.8). The decomposition of NH(4)Y(2)F(7) occurs by random nucleation and growth.